More particularly, the invention relates to an electronic digital-to-analog converter circuit for transforming a digital signal into an analog signal without phase distortion, and in particular a circuit comprising a sigma-delta type modulator and a filter having a finite impulse response.
A preferred field of application of the invention is the field of mobile telephony. The invention is described essentially in the context of this field. Nevertheless, the scope of the invention should not be limited to the field of mobile telephony, but should be understood as extending to any other field in which the converter circuit and the method of conversion of the invention are applied.
In general, a mobile telephone or a cordless telephone can be structurally subdivided into four distinct blocks: the user block or user interface; the control block; the audio block; and the radio frequency block. The last three blocks to be mentioned constitute the radio unit of the mobile telephone.
FIG. 1 is a block diagram showing the general structure 100 of a mobile telephone. The general structure 100 has four main blocks as mentioned above. The user of a mobile telephone has access to a user interface 120. The user interface 120 generally comprises a loudspeaker 121, a microphone 122, a keypad 123, and display means 124. The user interface 120 can optionally include other elements, for example a modem for transferring data.
A radio unit 130 comprises the blocks that are required for baseband transmission: speech encoding means; means for compressing information into data blocks; and means for decompressing such data blocks into a continuous signal. These elements, and others, are shared between a control block 140, an audio block 150, and a radio frequency block 160.
The control block 140 comprises a microprocessor 141 which acts as a central processor unit. The microprocessor 141 performs the procedures necessary for setting up a call. It also controls the various operations of the mobile telephone by means of various programs. These programs include, for example, programs for managing the user interface 120, monitoring programs (in particular for monitoring battery level), and test programs in order to facilitate maintenance of the mobile telephone. Other programs manage the connection between the mobile telephone and the nearest transmission relay. The programs associated with the user interface 120 manage, in particular, interactions between the user and the other programs, in particular by interpreting the information provided by the user to the microprocessor 141 via the digital keypad 123 and a link 101, and by controlling the display means 124 via a link 102.
The control block 140 also has memories 142 which are used in mobile telephones for storing, in particular, the operating system, the serial number, and the telephone number associated with the mobile telephone, or indeed rights to use various services. These memories 142 can also be used when setting up a call. Data information, memory addresses, and commands are interchanged between the memories 142 and the microprocessors 141 over a bidirectional bus 103.
The audio block 150 is essentially constituted by a signal processor unit 151 which makes use of numerous programs. The signal processor unit 151 receives information from the microphone 122 over a link 104. A link 105 serves to transmit signals between the signal processor unit 151 and the loudspeaker 121. Information is also interchanged between the signal processor unit 151 and the microprocessor 141 over a bidirectional link 106.
Communication with the radio unit 130 of the mobile telephone takes place by means of a special radio frequency interface 160 in which analog-to-digital and digital-to-analog conversions are performed. The radiofrequency block 160 comprises, in particular: an antenna 161 connected to a duplexer 162; a transmitter 163; a receiver 164; and a frequency generator unit 165. The microprocessor 141 manages the operation of the transmitter 163, the receiver 164, and the frequency generator 165 over respective connections 107, 108, and 109. The signal processor unit 151 can send signals to the transmitter 163 and can receive signals from the receiver 164 over respective links 110 and 111. The frequency generator 165 is connected to the transmitter 163 and to the receiver 164 over respective connections 112 and 113. The duplexer 162 receives signals from the transmitter 163 over a link 114 and sends signals to the receiver 164 over a link 115.
In the transmitter 163, signals carrying speech information and other information as required for telecommunication are modulated for transmission by means of a radiofrequency carrier wave. Modulation operations are usually performed at an intermediate frequency which is mixed with the desired transmission frequency. Various methods of modulation are known. They depend on the type of signal and on the equipment available for transmission. To transmit analog information, it is possible to use frequency modulation or frequency shift keying (FSK). To transfer digital information, it is possible to use phase shift keying (PSK) e.g. of the .pi./4 PSK type, or to use Gaussian minimum shift keying (GMSK).
FIG. 2 is a simplified block diagram of a transmission system for the transmitter 163, and more particularly of a prior art digital-to-analog converter circuit. The signal coming from the signal processor unit 151 is sent in the form of digital data to an interface 210 of the transmitter 163. The signal received by the transmitter 163 is processed in succession by the interface 210, by a modulator 220 performing GMSK type modulation, by a digital-to-analog converter 240 (DAC), by a sample-and-hold circuit 230, and by an analog filter 250.
In the prior art, for reasons to do with energy consumption and with ease of implementation, the DAC 240 is usually a switched-capacitor DAC. Thus, for example, each group of k bits from the modulator 220 produces a voltage that is proportional to the value encoded on the k bits directly at the output from the DAC 240. That method gives rise to problems of linearity between the signal from the DAC 240 and the signal input to the DAC 240. These problems of linearity are made worse by the fact that there is a sample-and-hold circuit 230 in the above-described transmission system. The DAC 240 is a switched-capacitor digital-to-analog converter. This means that for a half-period of a cyclic clock signal controlling the transfer of data along the above-described transmission system, the values of the bits processed by the DAC 240 are not available. The sample-and-hold circuit 230 is therefore required to maintain each processed bit value for at least one half-period of the clock signal. The presence of the sample-and-hold circuit 230 nevertheless increases problems of non-linearity in the transmission of the signal. In certain transmitters, and in particular in transmitters used in GSM type mobile telephone systems, the distortion of the transmitted signal can be very detrimental to the quality of the call.
The prior art, in particular patent EP A-0642221, describes an electronic digital-to-analog converter circuit for a baseband transmission system. The circuit includes an interpolator for increasing a sampling frequency of the digital signal, a sigma-delta type modulator, a digital-to-analog converter block comprising a finite impulse response filter (FIR), and finally an analog filter of lowpass filter type.
However, the FIR described in that document is a conventional FIR, i.e. having a transfer function of the type h(z)=1+a.sub.1 z.sup.-1 +a.sub.2 z.sup.-2 +a.sub.3 z.sup.-3 + . . . . Such a filter is not adapted to an application in the GSM field since it would require a large number of coefficients a.sub.1 resulting in an FIR of large volume and of high electricity consumption in order to obtain a filter having a steep slope.